Electrostatic or solid-state capacitors are energy storage devices ubiquitous in all major electrical and electronic devices and are an integrated component of Printed Circuit Boards (PCB), pulsed lasers, radars and sensors. These capacitors operate at a high voltage and have fast charge-discharge characteristics on the order of ms-μs unlike batteries and supercapacitors, which make them especially suitable for pulsed-power applications requiring high power densities.
Solid-state capacitors consist of a solid dielectric material between two electrodes, with the maximum energy storage density dependent on the dielectric properties of the material, namely relative permittivity (εr), breakdown strength (EBD) and dielectric loss (tan δ). Polymer films are widely employed as the dielectric material in these capacitors as they possess the advantages of being lightweight, easy to process and cheaper as compared to alternative ceramic dielectrics. Polymers also have inherently higher voltage endurance and lower dielectric losses. Although polymer film capacitors possess high power densities, they fall short in terms of energy storage density as compared to batteries, fuel cells or supercapacitors. There is a critical need to improve the current energy densities of polymer film capacitors to replace the bulky batteries and fuel cells for next-generation lightweight and flexible electronic systems.
A capacitor is charged by applying an electric field that is stored as energy within the dielectric material. One of the primary limiting factors for energy storage of a capacitor is voltage endurance i.e. breakdown strength (EBD) of the dielectric material at high electric fields or voltages. A primary mechanism of breakdown in polymer dielectrics is the formation of conducting fractal channels called electrical trees, which grow across the dielectric from a point of failure towards the electrode. If the propagation of applied electrical field through the electrical trees is impeded, it can significantly improve the breakdown strength of the dielectric and consequently the energy storage. This can be achieved through the use of new polymeric materials, architectures and processing strategies that enhance the dielectric material properties.